Cyclohexane Ring Flip and Boat Conformation

Cyclohexane Ring Flip and Boat Conformation


Leah here, from leah4sci.com and in this video
we’ll look at the ring flips or interconversion between cyclohexane chair conformations as
well as the boat intermediate. In the last video we learned about the basics
of the chair conformation as well as the concept of stability when it comes to axial versus
equatorial substituents. If i’m given a molecule as a flat hexagon,
I have an option of drawing two different chair conformations, if you’re not comfortable
with this drawing style follow along with the drawing tutorial link below. But the key here is to not even pay attention
to the molecule when you’re drawing your chairs, we’ll start by drawing two different versions
of the chair and then place the methyl group in the up position. I know that it’s up because methyl is on a
wedge and a wedge is coming up and out of the page directly at you, but the question
is “Do I prefer to put it as an axial up or equatorial up?”. Both are technically correct so which one
am I going to see for methylcyclohexane? What’s the difference between these two? In the first one, the methyl substituent is
axial, the second one has the methyl equatorial and since axial is less stable than equatorial,
equatorial would be the more favored structure. Does that mean that the molecule will exist
as an equatorial substituent at all times? Is this the chair conformation we’re going
to see in nature? When would we see this kind of conformation,
and the answer is that we’re going to see both. The chair structure can occur one way or the
other at random and they’ll actually exists in some sort of equilibrium because the chair
will flip from one version to the other. Ring flips are tricky if you simply memorize
what’s going on, so let’s switch over to the model kit and understand the nature of the
interconversion, understand what is happening with that ring flip and then I’ll show you
quickly do it on paper. Here we have cyclohexane and a random chair
conformation and as a quick reminder, the red substituents are what we see going up
and blue are what we see going down. I want you to pay special attention to how
these move when we do the ring flip. In order to do a ring flip, you start at any
point in the molecule, you can start with any of the carbons that are up, you could
start with carbons that are down, it doesn’t matter. Pick a convention and stick with it so that
it make sense. Take one carbon and I’ll start with the one
that’s pointing up and flip it down, then take the opposite carbons so if this is carbon
one, that would be two, three, and four, and flip that one up. This was initially an axial carbon that became
equatorial and this was initially an equatorial carbon that is now axial, now this one is
down, this one went up, this one went down, this one went up. Everything changed it’s position from this
ring flip so let’s do that one more time. Now that this one is up, we’re going to bend
it so it goes down, carbon on the opposite end, bend it and it goes up. Now let’s take a look at what happened, notice
that the red substituents started out up and no matter how I flip this ring, the red substituents
will always stay in the up position. Up will always stay up. Look at the blue substituents, no matter how
many times I flip it, they’re all in the down position. Substituents that start at going down are
going to remain going down, so what is it that changes in a chair conformation ring
flip? Let’s take a look at another chair and you’ll
remember this one from the last video, because it has the green substituents are all axial
and the white are all equatorial they are outside to the equator, watch what happens
when I do a ring flip here. We’ll start with this carbon that’s up and
drag it down, we’ll start with this carbon that’s down and drag it up, look what happened,
the green substituents are no longer axial, every single one became equatorial, the white
substituents that were pointing to the equator are now all axial pointing up and down and
the green ones are equatorial so even though up stays up and down stays down, substituents
that start out axial, will go equatorial after a ring flip. Substituents that start out equitorial, will
go axial after a ring flip. Now that you what changes and what stays the
same, let’s take a look at the steps within that ring flip transition. The first thing we do is take a substituent
that is up or down and bring it to the opposite direction, but as we bend it upward, watch
what happens. This thing now is flat, it’s not up, it’s
not down, it’s not a chair anymore, this part here, this is still a chair. This part is not a chair so we call this the
“Half-Chair”, it has to go through the half chair on it’s way up, but on it’s way up it
tends to bend to the side, because this confirmation here, has substituents getting in each other’s
way so it slightly bends to the side. This is called a “Twist Boat” but then it
has to untwist and get into this conformation which kind of looks like a canoe, this is
called the “Boat” conformation. With the boat conformation you can start twisting
the other direction so once again we get a twist boat as this starts to straighten out
into a half-chair and goes down for the other chair conformation. Again, we have the chair, half-chair, twist
boat, boat, twist boat again, half-chair, and a chair conformation. Let’s see what this looks like on paper. We start out with a stable chair conformation
and relatively low energy, remember cyclohexane is most stable in a chair conformation, but
in order to do a ring flip, we drag up this carbon. That means the left half of the molecule is
still going to look like a chair. This gives us the half chair because the left
side is flat, it’s not up, it’s not down, it’s half a chair, half a flat unstable thing. It’s unstable because these carbons have very
undesirable bond angles and that makes the molecules unhappy, it wants to get out of
that position as quickly as it can. In transitioning to a boat conformation we
have a twist boat, which is so much tricky to draw but you basically do an X and then
add the two terminal carbons. So these two lines, that means these 4 carbons
are represented here on the x and then these two carbons would be these two. The twist boat is more stable than the boat
because the two hydrogens that were facing each other on the boat conformation are not
as close as the boat itself but it’s still relatively unstable. So we’ll draw it lower than a half chair but
definitely higher than the chair which is the most stable structure. In order to transition into the other version
of the chair we have to undergo the unstable boat conformation which starts to look like
a chair again except that both sides have their substituents facing up. If we redraw it like this, you can start to
envision a canoe or a boat. Why a boat conformations so much less stable
than chairs? well look at what we have here. Most of the carbons seems pretty happy, the
bond angles look pretty good. But, at the very top of the boat, these two
carbons here, we have hydrogen atoms that are very close to each other, these are called
the flagpole hydrogens and they’re kind of getting in each other’s space. When atoms get to close to each other, when
they invade personal space it’s considered very, very unstable. This is why you get the twist boat because
the flagpole hydrogens want to go away from each other. It’s not as visible here but take a look from
the top, they’re very close to each other, very unstable, twist the boat and now they
have some space. Twist the boat, they have some space. This is what makes them more stable but ultimately
the molecules waiting for this. When you go into a full chair conformation,
everything is relatively far away and so much more stable. Now that we have a boat we can start transitioning
into the other chair conformation. Once again we do the twist boat, this time
we do the X in the opposite direction. I know it’s not fully visible here but if
you had substituents you could start showing axial going equatorial and equatorial going
axial. How do you know what to draw for the final
chair? well watch the progress. This carbon here was dragged up so that it’s
in the up position, that’s right here. That means we have to start dragging this
carbon, this carbon down. So, we re-draw the chair as we saw initially,
but now the carbon on the right is in the up position, the carbon on the left that is
on its way down is now planar and this gives us another half chair which allows us to continue
dragging that carbon all the way down for that final and very stable opposite chair
conformation. Why did I draw them this way on the screen? If you had to show this as an energy diagram,
you start out with a very stable chair and raise the energy all the way to the half chair. It goes down slightly for the twist boat,
higher in energy for the boat and click down for the twist boat so it can go all the way
up to the half chair so it can go all the way down to the stable chair conformation. Let’s apply this to the initial molecule that
we saw. It’s natural to look at the molecule, draw
a chair, analyze a chair, and attempt to flip it. But now that you understand what’s going on,
let’s switch over the model kit and apply the ring flip to our initial molecule. We were given a methyl cyclo hexane which
can be drawn as hexagon with the methyl or written as either of two chair conformations. How do we do the ring flip? I’d like to put my substituents on the up
corner, you can put it wherever you want. Remember this is a ring so you can go round
and round and round put it wherever you want as long as you stay consistent. We have the methyl group here in the chair
conformation with the up position. So we simply take that methyl, drag it down,
take the other end of the chair and drag it up. All the intermediates were there, so we went
from chair, half chair, twist boat, boat, twist boat again, other half chair and chair. But in short, like that one up, one down. Try this, it’s fun. Once you understand what is going to happen,
in other words, the methyl group starts out in the up position. the methyl group ends up in the up position. The only thing that changed is having the
axial, first is equatorial. Once you get that you don’t have to worry
about it when you’re drawing making the entire ring flip process so much easier. How so? If we’re trying to figure out what to draw,
we don’t even need our initial molecule to figure out what boat chair conformations will
look like. What you do instead is just draw two chair
conformations skeletons so that the parallel lines are opposite in nature. For the first structure, I have left over
right. For the second structure I’ll have right
over left. Again this is taught in the drawing chairs
tutorial linked below. Here’s the best part. Take your cyclohexane and number it so that
you know where the substituents are located. We only have a methyl so it’s pretty easy
put a number 1. Anywhere on this chair conformation, put a
number 1. So we put the 1 here. Now ask yourself, in order to do a ring flip,
what happened? Well, I chose to take that carbon number 1
and drag it down or you can start by taking carbon number 4 and dragging it up. It doesn’t matter as long as you’re consistent. If carbon 1 came down, this is that same carbon
number 1. Now that I know what’s what, I simply ask
myself, what substituent is where? Carbon number 1 has a methyl in the up position. Up position on the structure has to be axial,
up position on this structure has to be equatorial because the other substituent is down in this
case equatorial, here it’s axial. If we had a second substituent, you would
do the same thing. For example, if we have another methyl here
also in the up position. I would continue numbering two and three clockwise. This is important, I don’t care where you
start, I don’t care where you continue. But if your hexagon is clockwise, your chairs
have to be clockwise. If the hexagon is counter, the chairs have
to be counter. We’re going clockwise so we’ll go 2,3, so
we’ll draw 2, 3 and stop. We don’t need to number all the way through
6. 1,2,3, in a clockwise direction. Where is methyl on carbon 3? The up position, once again we have a wedge
is axial on the left structure and equatorial on the right structure. And since we have a gray line, let’s put a
hydrogen so you don’t get confused with the methyl groups and there we have it a quick
and simple ring flip that starts out by simply drawing both chairs figuring out where your
substituents are located and drawing them onto the molecule. Once again let’s figure out which one is more
stable. On the left we have two substituents that
are axial. On the right we have 2 substituents that are
equatorial. And since we know the axial substituents will
have unfavorable diaxial interactions, we know the left structure is less stable and
the right structure is more stable. One last thing I wanna share with chair conformations. Structures will typically exist in equilibrium
with each other. If we go back to the previous example, when
there is one methyl group, the equilibrium will tend to favor equatorial, slightly more
than axial. When there are two methyl groups, equilibrium
will favor this one much more because the more the difference between stability, the
more time the molecule will spend in the more stable position. Two equatorial is so much more stable so the
molecule will tend to favor this structure but it’ll still go back and forth, you will
still see that ring happening again and again . But if you have a molecule that has a tert-butyl
substituent which is a very very big bulky group, that’s a C (CH3)3. It’s like carrying a giant umbrella. This molecule is going to spend 99% of it’s
time where the tert-butyl group is in the equatorial substituent. You can think of this as a tert-butyl group
locks the ring equatorial and that means anything you have is going to remain locked in that
position. Keep this in mind when you get to reactions
especially,especially when you’re doing E2 reactions on cyclohexane. Ready to try some practice problems? You can find the entire chair conformation
tutorial series along with the practice quiz on my website leah4sci.com/chairs.

34 thoughts on “Cyclohexane Ring Flip and Boat Conformation

  1. Amazing… Explanation. The concept was so confusing before and now I have full clarity. And the greater part is you made it effortless…

  2. Teacher I didnt understand that how ccan the stability of half chair be less than the boat?Because , you drew energy diagram of half chair more higher than every other conformation ?

  3. Very well explained. But I still have a doubt. When you do a ring flip, the subtituents which are in up position remain up but the carbon bearing the same substituents goes down….am I right kindly clarify

  4. Hello mam very much impressed with your video, one small request, can you please provide me the link where you had purchased the atomic Model, I want the same atomic model

  5. Thank you mam for this superb video. I was really very confused with this topic. You made it look so simple for me. I am preparing for IIT examination.

  6. Thanks a lot💙…… this video is so helpful for every higher level students who reads in bepartment of chemistry….
    I am form Bangladesh

Leave a Reply

Your email address will not be published. Required fields are marked *